Thin cast strip product with microalloy additions, and method for making the same

ABSTRACT

A steel product or thin steel cast strip comprised of, by weight, less than 0.25% carbon, between 0.20 and 2.0% manganese, between 0.05 and 0.50% silicon, less than 0.06% aluminum, and at least one element selected from the group consisting of titanium between about 0.01% and about 0.20%, niobium between about 0.01% and about 0.20%, molybdenum between about 0.05% and about 0.50%, and vanadium between about 0.01% and about 0.20%, and having a microstructure comprised of a majority bainite, and further comprising fine oxide particles of silicon and iron distributed through the steel microstructure having an average precipitate size less than 50 nanometers. The steel product or thin cast steel strip may have a yield strength of at least 55 ksi (380 MPa) or a tensile strength of at least 500 MPa, or both. The steel product or thin cast steel strip may have a total elongation of at least 6% or 10%. The thin cast steel strip may have thickness less than 3.0 mm, or less than 2.5 mm, or less than 2.0 mm.

RELATED APPLICATIONS

This application is a continuation-in-part application of applicationSer. No. 11/255,604, filed Oct. 20, 2006.

BACKGROUND AND SUMMARY

This invention relates to making of high strength thin cast strip, andthe method for making such cast strip by a twin roll caster.

In a twin roll caster, molten metal is introduced between a pair ofcounter-rotated, internally cooled casting rolls so that metal shellssolidify on the moving roll surfaces, and are brought together at thenip between them to produce a solidified strip product, delivereddownwardly from the nip between the casting rolls. The term “nip” isused herein to refer to the general region at which the casting rollsare closest together. The molten metal is poured from a ladle through ametal delivery system comprised of a tundish and a core nozzle locatedabove the nip to form a casting pool of molten metal, supported on thecasting surfaces of the rolls above the nip and extending along thelength of the nip. This casting pool is usually confined betweenrefractory side plates or dams held in sliding engagement with the endsurfaces of the rolls so as to dam the two ends of the casting poolagainst outflow.

In the past, high-strength low-carbon thin strip with yield strengths of60 ksi (413 MPa) and higher, in strip thicknesses less than 3.0 mm, havebeen made by recovery annealing of cold rolled strip. Cold rolling wasrequired to produce the desired thickness. The cold roll strip was thenrecovery annealed to improve the ductility without significantlyreducing the strength. However, the final ductility of the resultingstrip still was relatively low and the strip would not achieve totalelongation levels over 6%, which is required for structural steels bybuilding codes for structural components. Such recovery annealed coldrolled, low-carbon steel was generally suitable only for simple formingoperations, e.g., roll forming and bending. To produce this steel stripwith higher ductility was not technically feasible in these final stripthicknesses using the cold rolled and recovery annealed manufacturingroute.

In the past, high strength, low carbon steel strip have also been madeby microalloying with elements such as niobium, vanadium, titanium ormolybdenum, and hot rolling to achieve the desired thickness andstrength level. Such microalloying required expensive and high levels ofniobium, vanadium, titanium or molybdenum and resulted in formation of abainite-ferrite microstructure typically with 10 to 20% bainite. SeeU.S. Pat. No. 6,488,790. Alternately, the microstructure could beferrite with 10-20% pearlite. Hot rolling the strip resulted in thepartial precipitation of these alloying elements. As a result,relatively high alloying levels of the Nb, V, Ti or Mo elements wererequired to provide enough precipitation hardening of the predominatelyferritic transformed microstructure to achieve the required strengthlevels. These high microalloying levels significantly raised the hotrolling loads needed and restricted the thickness range of the hotrolled strip that could be economically and practically produced. Suchalloyed high strength strip could be directly used for galvanizing afterpickling for the thicker end of the product range greater than 3 mm inthickness.

However, making of high strength, low carbon steel strip less than 3 mmin thickness with microalloying additions of Nb, V, Ti or Mo to the basesteel chemistry was very difficult, particularly for wide strip due tothe high rolling loads, and not always commercially feasible. For lowerthicknesses of strip, cold rolling was required; however, the highstrength of the hot rolled strip made such cold rolling difficultbecause of the high cold roll loadings required to reduce the thicknessof the strip. These high alloying levels also considerably raised therecrystallization annealing temperature needed, requiring expensive tobuild and operate annealing lines capable of achieving the highannealing temperature needed for full recrystallization annealing of thecold rolled strip.

In short, the application of previously known microalloying practiceswith Ni, V, Ti or Mo elements to produce high strength thin strip couldnot be commercially produced economically because of the high alloyingcosts, difficulties with high rolling loads in hot rolling and coldrolling, and the high recrystallization annealing temperatures required.

The invention presently disclosed is a steel product comprised, byweight, of less than 0.25% carbon, between 0.2 and 2.0% manganese,between 0.05 and 0.5% silicon, less than 0.06% aluminum, and at leastone element selected from the group consisting of titanium between about0.01% and about 0.20%, niobium between about 0.01% and about 0.20%,molybdenum between about 0.05% and about 0.50%, and vanadium betweenabout 0.01% and about 0.20%, and having a majority of the microstructurecomprised of bainite and fine oxide particles containing silicon andiron distributed through the steel microstructure having an averageprecipitate size less than 50 nanometers. The steel product may befurther comprised of a more uniform distribution of microalloys throughthe microstructure than previously produced with conventional slab castproduct.

Alternatively or in addition, the low carbon steel product may have atotal elongation greater than 6% or greater than 10%. The steel productmay have yield strength of at least 55 ksi (380 MPa) or a tensilestrength of at least 500 MPa, or both.

In addition, a thin cast strip is disclosed comprising, by weight, lessthan 0.25% carbon, between 0.20 and 2.0% manganese, between 0.05 and0.50% silicon, less than 0.06% aluminum, and between about 0.01% andabout 0.20% niobium, and having a microstructure comprised of a majorityof bainite. The thin cast strip may have fine oxide particles of siliconand iron distributed through the steel microstructure having an averageprecipitate size less than 50 nanometers. The steel product may befurther comprised of a more uniform distribution of microalloys throughthe microstructure than previously produced with conventional slab castproduct.

The thin cast strip may a thickness less than 3 mm, or less than 2.5 mm,or less than 2 mm down to as thin as commercially feasible. The thincast strip may have a thickness in the range from about 0.5 mm to about2 mm. The thin cast strip may have a total elongation greater than 6% orgreater than 10%. The steel product may have yield strength of at least55 ksi (380 MPa) or a tensile strength of at least 500 MPa, or both.

In addition, a method is disclosed of preparing a thin cast steel stripcomprising the steps of:

assembling a roll caster having laterally positioned casting rollsforming a nip between them, and forming a casting pool of molten lowcarbon steel supported on the casting rolls above the nip and confinedadjacent the ends of the casting rolls by side dams,

counter rotating the casting rolls to solidify metal shells on thecasting rolls as the rolls move through the casting pool; forming fromthe metal shells downwardly through the nip between the casting rolls asteel strip; and

cooling the steel strip at a rate above 10° C. per second to produce asteel strip having a composition comprising by weight, less than 0.25%carbon, between 0.50 and 2.0% manganese, between 0.05 and 0.50% silicon,less than 0.06% aluminum, and between about 0.01% and about 0.20%niobium, and having a microstructure with a majority comprised ofbainite.

The steel strip as coiled may have fine oxide particles of silicon andiron distributed through the steel microstructure having an averageprecipitate size less than 50 nanometers.

The method of preparing a thin cast steel strip may further comprise thesteps of:

hot rolling the low carbon steel strip; and

coiling the hot rolled low carbon steel strip at a temperature in therange from about 500-700° C.

The method of preparing a thin cast steel strip may also comprise thesteps of:

precipitation hardening the low carbon steel strip to increase thetensile strength at a temperature of at least 550° C.

The precipitation hardening may occur at a temperature between 650° C.and 800° C. or between 675° C. and 750° C.

The precipitation hardening may occur during the processing of the stripthrough a galvanizing line or continuous annealing line, or other heattreating process.

BRIEF DESCRIPTION OF THE DRAWINGS

In order that the invention may be described in more detail, someillustrative examples will be given with reference to the accompanyingdrawings in which:

FIG. 1 illustrates a strip casting installation incorporating an in-linehot rolling mill and coiler;

FIG. 2 illustrates details of the twin roll strip caster;

FIG. 3 illustrates the effect of coiling temperature on strip yieldstrength with and without microalloy additions;

FIG. 4 a is an optical micrograph of a microalloyed steel strip;

FIG. 4 b is an optical micrograph of a standard UCS SS Grade 380 steelstrip;

FIG. 5 is graph showing the effect of post coil heat treatment on yieldstrength of a microalloyed steel strip; and

FIG. 6 is a graph showing the effect of post coiling simulated heattreatment cycle on yield and tensile strength of a microalloyed steelstrip.

DETAILED DESCRIPTION OF THE DRAWINGS

The following description of the embodiments is in the context of highstrength thin cast strip with microalloy additions made by continuouscasting steel strip using a twin roll caster. The embodiments describedherein are not limited to the use of twin roll casters and extends toother types of continuous strip casters.

FIG. 1 illustrates successive parts of strip caster for continuouslycasting steel strip. FIGS. 1 and 2 illustrate a twin roll caster 11 thatcontinuously produces a cast steel strip 12, which passes in a transmitpath 10 across a guide table 13 to a pinch roll stand 14 having pinchrolls 14A. Immediately after exiting the pinch roll stand 14, the strippasses into a hot rolling mill 16 having a pair of reduction rolls 16Aand backing rolls 16B where the cast strip is hot rolled to reduce adesired thickness. The hot rolled strip passes onto a run-out table 17where the strip may be cooled by convection and contact with watersupplied via water jets 18 (or other suitable means) and by radiation.The rolled and cooled strip is then passes through a pinch roll stand 20comprising a pair of pinch rolls 20A and then to a coiler 19. Finalcooling of the cast strip takes place after coiling.

As shown in FIG. 2, twin roll caster 11 comprises a main machine frame21 which supports a pair of laterally positioned casting rolls 22 havingcasting surfaces 22A. Molten metal is supplied during a castingoperation from a ladle (not shown) to a tundish 23, through a refractoryshroud 24 to a distributor or moveable tundish 25, and then from thetundish 25 through a metal delivery nozzle 26 between the casting rolls22 above the nip 27. The molten metal delivered between the castingrolls 22 forms a casting pool 30 above the nip. The casting pool 30 isrestrained at the ends of the casting rolls by a pair of side closuredams or plates 28, which are pushed against the ends of the castingrolls by a pair of thrusters (not shown) including hydraulic cylinderunits (not shown) connected to the side plate holders. The upper surfaceof casting pool 30 (generally referred to as the “meniscus” level)usually rises above the lower end of the delivery nozzle so that thelower end of the delivery nozzle is immersed within the casting pool 30.Casting rolls 22 are internally water cooled so that shells solidify onthe moving roller surfaces as they pass through the casting pool, andare brought together at the nip 27 between them to produce the caststrip 12, which is delivered downwardly from the nip between the castingrolls.

The twin roll caster may be of the kind which is illustrated anddescribed in some detail in U.S. Pat. Nos. 5,184,668 and 5,277,243 orU.S. Pat. No. 5,488,988. Reference may be made to those patents forappropriate construction details of a twin roll caster appropriate foruse in an embodiment of the present invention.

A high strength thin cast strip product can be produced using the twinroll caster that overcomes the shortcomings of conventional light gaugesteel products and produces a high strength, light gauge, low carbon,steel strip product. Low carbon steel here refers to steels having acarbon level below 0.1% by weight. The invention utilizes themicroalloying elements including niobium, vanadium, titanium ormolybdenum or a combination thereof.

Microalloying elements in steel are commonly taken to refer to theelements titanium niobium, and vanadium. These microalloying elementswere usually added in the past in levels below 0.1%, but in some caseslevels as high as 0.2%. These microalloying elements are capable ofexerting strong effects on the steel microstructure and properties via acombination of hardenability, grain refining and precipitationstrengthening effects (in the past as carbonitride formers). Molybdenumhas not normally regarded as a microalloying element since on its own itis a relatively weak carbonitride former, but in the presentcircumstances carbonitdride formation is inhibited in the hot rolledstrip with these microalloys as explained below.

The high strength thin cast strip product combines several attributes toachieve a high strength light gauge cast strip product by microalloyingwith these elements. Strip thicknesses may be less than 3 mm, less than2.5 mm, or less than 2.0 mm, and may be in a range of 0.5 mm to 2.0 mm.The cast strip is produced by hot rolling without the need for coldrolling to further reduce the strip to the desired thickness. Thus, thehigh strength thin cast strip product overlaps both the light gauge hotrolled thickness ranges and the cold rolled thickness ranges desired.The strip may be cooled at a rate of 10° C. per second and above, andstill form a microstructure that is a majority and typicallypredominantly bainite.

The benefits achieved through the preparation of such a high strengththin cast strip product are in contrast to the production of previousconventionally produced microalloyed steels which results in relativelyhigh alloy costs, difficulties in hot and cold rolling and difficultiesin recrystallation annealing since conventional continuous galvanizingand annealing lines are not capable of providing the high annealingtemperatures needed. Moreover, the relatively poor ductility exhibitedwith strip made the cold rolled and recovery annealed manufacturingroute is overcome.

The high strength thin cast steel strip product was produced comprising,by weight, less than 0.25% carbon, between 0.20 and 2.00% manganese,between 0.05 and 0.50% silicon, less than 0.06% aluminum, and at leastone element selected from the group consisting of titanium between about0.01% and about 0.20%, niobium between about 0.01% and about 0.20%,molybdenum between about 0.05% and about 0.50%, and vanadium betweenabout 0.01% and about 0.20%, and having a microstructure comprising amajority bainite. The steel product may further comprising fine oxideparticles of silicon and iron distributed through the steelmicrostructure having an average precipitate size less than 50nanometers. The steel product may be further comprised of a more uniformdistribution of microalloys through the microstructure than previouslyproduced with conventional slab cast product.

Alternatively or in addition, the low carbon steel product may have atotal elongation greater than 6% or greater than 10%. The steel productmay have a yield strength of at least 55 ksi (380 MPa) or a tensilestrength of at least 500 MPa, or both.

After hot rolling the hot rolled low carbon steel strip may be coiled ata temperature in the range from about 500-700° C. The thin cast steelstrip may also be further processed by precipitation hardening the lowcarbon steel strip to increase the tensile strength at a temperature ofat least 550° C. The precipitation hardening may occur at a temperaturebetween 550° C. and 800° C. or between 675° C. and 750° C. Conventionalfurnaces of continuous galvanizing or annealing lines are thus capableof providing the precipitation hardening temperatures needed to hardenthe microalloyed cast strip product.

For example, a steel composition was prepared by making a steelcomposition of a 0.026% niobium, 0.04% by weight carbon, 0.85% by weightmanganese, 0.25% by weight silicon that has been cast by a thin caststrip process. The strip was cast at 1.7 mm thick and inline hot rolledto a range of strip thickness from 1.5 mm to 1.1 mm using a twin rollcaster as illustrated in FIGS. 1 and 2. The strip was coiled at coilingtemperatures of 590-620° C. (1094-1148° F.).

As shown in FIG. 3, the yield and tensile strength levels achieved inthe microalloyed cast strip are compared to the yield and tensilestrength levels achievable in the base, non-microalloyed, cast stripsteel composition over a range of coiling temperatures. It can be seenthat the niobium microalloyed steel strip achieved yield strengths inthe range of 420-440 MPa (˜61-64 ksi) and tensile strengths of about 510MPa (˜74 ksi). The Nb cast strip product is compared to C—Mn—Si basesteel compositions processed with the same coiling temperature as theniobium microalloyed steel, and the niobium microalloyed steel producedsubstantially higher strength levels. The compared steel strip had to becoiled at very low temperatures to approach comparable strength levelsto the cast niobium microalloyed steel product. The cast niobium steelproduct did not need to be coiled at low coiling temperatures to achieveits strengthening potential with the hot rolling. Moreover, the yieldand tensile strength levels for the cast niobium microalloyed steel wasnot significantly affected by the degree of inline hot rolling with areduction of 19 to 37%.

The thin cast strip niobium steel product had consistent yield andtensile strength levels over the range of hot rolling applied during thetrial (reduction 19 to 37%). The prior austenite grain size wasdetermined for each strip thickness. The austenite grain sizemeasurements indicated that only very limited recrystallization hadoccurred at high hot rolling reductions, whereas in the comparable basesteel strip, the microstructure almost fully recrystallized at hotrolling reductions over about 25%. The addition of the microalloyingelement niobium to the cast steel strip suppressed the recrystallizationof the coarse as-cast austenite grain size during the hot rollingprocess, and resulted in the hardenability of the steel being retainedafter hot rolling.

The higher strength of the niobium microalloyed steel strip after hotrolling was mostly due to the microstructure formed. As shown in FIG. 4a, the microstructure of the cast niobium steel was comprised of amajority if not mostly bainite for all strip thicknesses. In contrast,as shown in FIG. 4 b, the comparable non-microalloyed steel achievedsimilar strength by coiling at a low coiling temperature and had amicrostructure comprising mostly acicular ferrite with some grainboundary ferrite. The microalloy addition of niobium to the steel stripprovided an increase in the hardenability of the steel and suppressedthe formation of the grain boundary ferrite and promoted the bainiticmicrostructure, even at considerably higher coiling temperatures.

In addition, transmission electron microscopy (TEM) examination did notreveal any substantial niobium precipitation in the as hot rolled caststrip. This indicates that the niobium had been retained in solidsolution and that the strengthening produced was mainly attributed tothe enhanced hardenability effect of the niobium resulting in theformation of a majority and likely predominantly bainiticmicrostructure. The hardenability of the cast steel strip is alsobelieved to be enhanced by the retention of coarse austenite grainproduced during formation of the cast strip. The transformation tobainite, rather than ferrite, is believed to be a major factor insuppressing the precipitation of the microalloy addition of niobium inthe thin cast strip during cooling of the coil from the coilingtemperature.

An additional factor believed to account for the absence of niobium richprecipitates in the hot rolled cast strip relates to the nature of thedispersion of niobium with the rapid solidification of the strip duringits formation by the method of continuously making cast strip described.In previously made microalloyed high strength strip, relatively longtime intervals were involved in the solidification with slab cooling,slab reheating and thermo-mechanical processing that permittedopportunities for pre-clustering and/or solid state precipitation ofmicroalloy carbonitride particles such as (Nb,V,Ti,Mo)(CN) that enabledthe kinetics for subsequent precipitation in various stages of themanufacturing process. In the process described, where the cast strip iscontinuously formed from a casting pool between casting rolls, theextremely rapid initial solidification in forming the cast strip (inabout 160 microseconds) is believed to inhibit pre-clustering and/orsolid state precipitation of microalloy carbonitride particles, and inturn, slow and reduce the kinetics for precipitation of the microalloysin subsequent processing including rolling and coiling operations. Thismeans that the microalloys of Nb, V, Ti, and Mo are relatively moreuniformly distributed in the austenite and ferrite phases, than in thinsteel strip previously made by conventional slab casting and processing.

Atom probe analysis of Nb microalloyed cast strip made by forming from acasting pool between casting rolls as above described has verified themore uniform distribution of microalloys (indicating reducedpre-clustering and/or solid state precipitation) in both the as cast andthe hot rolled strip when coiled at about 600° C. or lower. This moreuniform distribution of microalloys is believed to be inhibiting theprecipitation of microalloy carbonitrides in the coiling operation underconditions where fine coherent precipitation are of such microalloysoccurred in previous conventionally made and processed microalloyed slabcast steel. The reduction or absence of pre-clustering and/or solidstate precipitation of carbonitrides in the Nb microalloyed cast stripmade by forming from a casting pool between casting rolls also slows thekinetics of precipitation of microalloys during subsequentthermo-mechanical processing such as annealing. This then permits theopportunity for precipitation hardening at temperatures higher thanthose where the particles in previously conventionally processed striplost their strengthening capacity through coarsening (Ostwald ripening)mechanisms.

Laboratory ageing heat treatments were then conducted at varioustemperatures and times to induce precipitation of the niobium, that wasbelieved retained in solid solution in the hot rolled strip. As shown inFIG. 5, ageing heat treatments produced a significant increase instrength, with yield strengths of about 480 MPa (˜70 ksi). Thisconfirmed that the niobium was retained in a solid solution and wasavailable to provide precipitation hardening on subsequent ageing, forexample, through the use of an annealing furnace on continuousgalvanizing lines or by using a continuous annealing line. Accordingly,short time ageing heat treatments were carried out to simulate theageing potential from processing the niobium microalloyed cast steelproduct through an annealing furnace attached to continuous galvanizingline or conventional continuous annealing line. In the latter case theprecipitation hardened high strength strip product maybe subsequentlygalvanized, painted or utilized uncoated.

The results, as shown in FIG. 6, clearly show that for a peak processingtemperature of 700° C. (1292° F.), significant precipitationstrengthening was realized, with strength levels approaching thatachieved for the longer times at lower temperatures. The tensileproperties of the niobium microalloyed thin cast steel product after theshort time ageing treatment using a peak temperature of 700° C. (1292°F.) are given in Table 1. Besides the high strength of the cast stripproduct, the ductility and formability is satisfactory for structuralquality products. The cast strip product produced is a thin, highstrength strip product for structural applications through the use ofniobium microalloying. It is contemplated that higher microalloyinglevels would realize even higher yield strengths, potentially well inexcess of 550 MPa (˜80 ksi). TABLE 1 Strip Yield Tensile TotalThickness, Strength, Strength, Elongation, ‘n’ ‘r’ mm MPa MPa % YS/TSValue Value 1.1 477 563 18 0.85 0.12 0.90

Thus, it has been shown that the microalloyed Nb cast strip results inlight gauge, high strength, steel product. The Nb addition firstly iscapable of suppressing the austenite recrystallization during hotrolling which enhances the hardenability of the steel by retaining therelatively coarse as cast austenite size. The Nb being retained in solidsolution in austenite after hot rolling, then directly increases thesteel's hardenability, which assists in transforming the austenite to afinal microstructure comprised mostly of bainite, even at relativelyhigh coiling temperatures. The formation of a bainitic microstructurepromoted the retention of the Nb addition in solid solution in the hotrolled strip. Furthermore it was determined that the retention of theniobium in solid solution by the prior processing conditions, providedconsiderable precipitation hardening during a subsequent ageing heattreatment cycle. Such a heat treatment cycle can be produced using asuitable continuous galvanizing line or continuous annealing facility.Hence a microalloyed steel strip made using a thin strip castingprocess, combined with an ageing hardening heat treatment provided by asuitable galvanizing line or annealing line, is a unique manufacturingpath providing a unique strengthening approach for this type of steelproduct.

With a precipitation hardening heat treatment, an even higher tensilestrength was found to be achievable. For example, with a 0.026% niobiumaddition, an increase of at least a 5 ksi increase in yield strengthfrom 60-65 ksi was observed. With a 0.05% niobium addition, it iscontemplated that with a precipitation hardening heat treatment, anincrease of at least 10 ksi is expected, and a with 0.1% niobiumaddition, it is contemplated that with a precipitation hardening heattreatment, an increase of at least 20 ksi is expected. An annealingfurnace may be used to induce the precipitation hardening heattreatment, which is not a current strengthening approach for processingsuch products. The annealing condition may be a continuous annealingcycle with a peak temperature of at least 650° C. and less than 800° C.and better 675° C. to 750° C.

Similar results are contemplated with niobium between about 0.01% andabout 0.20%, as well as with titanium between about 0.01% and about0.20%, molybdenum between about 0.05% and about 0.50%, and vanadiumbetween about 0.01% and about 0.20%.

This microalloyed thin cast strip enables production of new steelproduct types including:

1. A high strength, light gauge, galvanized strip by utilizing amicrostructure that has bainite as the major constituent and agehardening during the galvanizing process. The annealing section of thegalvanizing line can be used to induce precipitation hardening of themicroalloying elements of the thin cast strip that has been hot rolled.

2. A high strength, light gauge, uncoated strip by utilizing amicrostructure that is majority bainite and age hardened duringprocessing on a continuous annealing line. The high temperature furnaceof the conventional continuous annealing can be used to induceprecipitation of the microalloying elements retained in solid solutionby the bainite microstructure after hot rolling of the thin cast strip.

3. A high strength, light gauge, hot rolled cast strip product where thestrength levels are insensitive to the degree of hot rolling reductionapplied. The bainitic microstructure produces a relatively high strengthproduct (YS≧380 MPa (˜55 ksi)). The suppression of austeniterecrystallization during or after hot rolling can provide final strengthlevels insensitive to the degree of hot rolling reduction. The finalstrength levels will be consistent across a range of thicknesses thatcan be produced by a thin cast strip process.

While the invention has been illustrated and described in detail in theforegoing drawings and description, the same is to be considered asillustrative and not restrictive in character, it being understood thatonly illustrative embodiments thereof have been shown and described, andthat all changes and modifications that come within the spirit of theinvention described by the following claims are desired to be protected.Additional features of the invention will become apparent to thoseskilled in the art upon consideration of the description. Modificationsmay be made without departing from the spirit and scope of theinvention.

1. A steel product comprising, by weight, less than 0.25% carbon,between 0.20 and 2.0% manganese, between 0.05 and 0.50% silicon, lessthan 0.06% aluminum, and at least one element selected from the groupconsisting of titanium between about 0.01% and about 0.20%, niobiumbetween about 0.01% and about 0.20%, molybdenum between about 0.05% andabout 0.50%, and vanadium between about 0.01% and about 0.20% and havinga majority of the microstructure comprised of bainite and comprisingfine oxide particles of silicon and iron distributed through the steelmicrostructure having an average precipitate size less than 50nanometers.
 2. The steel product as claimed in claim 1 wherein the steelproduct has a yield strength of at least 55 ksi (380 MPa).
 3. The steelproduct as claimed in claim 1 wherein the steel product has a tensilestrength of at least 72 ksi (500 MPa).
 4. The steel product as claimedin claim 1 wherein the steel product has a total elongation of at least6%.
 5. The steel product as claimed in claim 1 wherein the steel producthas a total elongation of at least 10%.
 6. A thin cast steel stripcomprising, by weight, less than 0.25% carbon, between 0.20 and 2.0%manganese, between 0.05 and 0.50% silicon, less than 0.06% aluminum, andat least one element selected from the group consisting of titaniumbetween about 0.01% and about 0.20%, niobium between about 0.01% andabout 0.20%, molybdenum between about 0.05% and about 0.50%, andvanadium between about 0.01% and about 0.20% and having a majority ofthe microstructure comprised of bainite.
 7. The thin cast strip asclaimed in claim 6 comprising in addition having fine oxide particles ofsilicon and iron distributed through the steel microstructure having anaverage precipitate size less than 50 nanometers.
 8. The thin cast steelstrip as claimed in claim 6 wherein the steel product has a yieldstrength of at least 55 ksi (380 MPa).
 9. The thin cast steel strip asclaimed in claim 6 wherein the steel product has a tensile strength ofat least 72 ksi (500 MPa).
 10. The thin cast steel strip as claimed inclaim 6 wherein the thin cast steel strip has a thickness of less than3.0 mm.
 11. The thin cast steel strip as claimed in claim 6 wherein thethin cast steel strip has a thickness of less than 2.5 mm.
 12. The thincast steel strip as claimed in claim 6 wherein the thin cast steel striphas a thickness of less than 2.0 mm.
 13. The thin cast steel strip asclaimed in claim 6 wherein the thin cast steel strip has a thickness inthe range from about 0.5 mm to about 2 mm.
 14. The thin cast steel stripas claimed in claim 6 wherein the steel product has a total elongationof at least 6%.
 15. The thin cast steel strip as claimed in claim 6wherein the steel product has a total elongation of at least 10%.
 16. Amethod of preparing a thin cast steel strip comprising the steps of:assembling internally a cooled roll caster having laterally positionedcasting rolls forming a nip between them, and forming a casting pool ofmolten low carbon steel supported on the casting rolls above the nip andconfined adjacent the ends of the casting rolls by side dams, counterrotating the casting rolls to solidify metal shells on the casting rollsas the rolls move through the casting pool; and forming from the metalshells downwardly through the nip between the casting rolls a steelstrip; and cooling the steel strip at a rate of 10° C. per second andabove to provide a composition comprising by weight, less than 0.25%carbon, between 0.20 and 2.0% manganese, between 0.05 and 0.50% silicon,less than 0.06% aluminum, and between about 0.01% and about 0.20%niobium, and having a microstructure with a majority comprised ofbainite.
 17. The method of preparing a thin cast steel strip as claimedin claim 16 wherein steel strip as coiled may have fine oxide particlesof silicon and iron distributed through the steel microstructure havingan average precipitate size less than 50 nanometers.
 18. The method ofpreparing a thin cast steel strip as claimed in claim 16 furthercomprise the steps of: hot rolling the molten low carbon steel strip;and coiling the hot rolled low carbon steel strip at a temperature inthe range from about 500-700° C.
 19. The method of preparing a thin caststeel strip as claimed in claim 18 further comprise the steps of:precipitation hardening the low carbon steel strip to increase thetensile strength at a temperature of at least 550° C.
 20. The method ofpreparing a thin cast steel strip as claimed in claim 18 furthercomprise the steps of: precipitation hardening may occur at atemperature between 650° C. and 800° C.
 21. The method of preparing athin cast steel strip as claimed in claim 18 further comprise the stepsof: precipitation hardening may occur at a temperature between 675° C.and 750° C.